Inside a circuit, a bit is a voltage on a wire. A logic level is the range of voltages that counts as one value:
- Low: near 0 V. Read as 0.
- High: near the supply voltage. Read as 1. The supply might be about 1 V inside a modern processor, or 3.3 V or 5 V on a hobby board.
Each level is a range, not an exact voltage. On a 3.3 V board, a wire at 0.3 V is still low and one at 3.0 V is still high. A band in the middle is not allowed, because a gate can't tell which value is meant.
Why ranges matter: electrical noise nudges voltages slightly, but not out of their range, so the bit survives. Every gate also outputs a fresh, full-strength level, so small errors don't build up from gate to gate. This is the main reason digital circuits are so reliable.
On a timing diagram, a line drawn high is a 1 and a line drawn low is a 0. The switch from low to high is a rising edge.
This course uses the usual convention, called positive logic, where high means 1. A few systems use the reverse, so datasheets always state which voltage means what.